This invention relates to coupling elements for slide fasteners, and in particular to the improved configuration of a continuous coupling element having a dual row of loops which can be fastened to a stringer tape as by stitching to provide a slide fastener half.
Continuous coupling elements for slide fasteners have been proposed and used in various configurations. Perhaps the most popular of such continuous coupling elements are those generally known as the coil and zigzag types. The continuous coupling elements of these well known types usually consist of elongated synthetic resin monofilaments which are shaped into a helical or zigzag configuration including a row of uniformly spaced apart coupling heads adapted for mating engagement with a similar row of heads of a complementary coupling element. A pair of legs or shanks extend rearwardly from each coupling head and are joined to arcuate or U-shaped bights bridging the adjacent shanks.
When the continuous coupling elements of these types are stitched onto stringer tapes as by a sewing machine, the elements must be retained in place on the tapes only by the threads extending longitudinally of the elements across their shanks. With the coupling elements fastened to the stringer tapes in this manner, there is no means for restraining them from lateral displacement. As a consequence, the prior art continuous coupling elements of the character described are prone to shift laterally out of their proper position when stitched to the stringer tapes.
In order to overcome this defect, there has been advanced another type of continuous coupling element having a multiplicity of undulations each consisting essentially of first and second loops. A row of coupling heads are formed along one longitudinal side of the first loops, and the second loops are arranged along the opposite longitudinal side of the first loops. These second loops constitute the support portion of this continuous coupling element, such that the same is stitched onto a stringer tape with the needle guided centrally through the successive second loops.
With this latter type of continuous coupling element, however, the row of stitches is located substantially the farthest transverse distance away from the coupling heads, so that the element cannot necessarily be stably anchored in position on the stringer tape. It is therefore highly likely that parts of the coupling elements should displace out of their correct relative positions in the act of coupling or uncoupling of the slide fastener stringers. In order to realize a high degree of coupling efficiency in slide fasteners, it is of absolute necessity that the individual undulations of the continuous coupling elements be failthfully maintained in their correct relative positions on the stringer tapes.